1. Field of Invention
This invention relates generally to optical coupling devices. More specifically, the invention concerns an improved method and apparatus for coupling one or more electrical signals between an optoelectronic device and one or more optical waveguide members.
2. Description of Related Art
One of the most important considerations in designing an electronic system is the connection between circuit components such as integrated circuits or optoelectronic devices. Sometimes, connections are needed between a first component mounted upon a printed circuit board and a second component located on the same board. Occasionally, connection is desired between a component mounted upon a printed circuit board and a connector located at the edge of the same board.
Traditionally, such connections have been accomplished by utilizing electrically conductive wiring. Such wiring is usually made from a metallic substance, and is often satisfactory for its intended purposes. However, there are often a number of disadvantages associated with the use of conventional metallic wiring. For example, metallic wiring is known to produce parasitic capacitance and inductance which can adversely affect a circuit's operation. In addition, metallic wiring produces electromagnetic noise which can interfere with the intended operation of a circuit. Furthermore, if metallic wiring is used more input/output lines might be required than might be desired due to the bandwidth limitations of metallic wiring.
One approach which has been used to overcome some of the disadvantages of conventional wiring is the use of light transmissive materials such as fiber optics. Fiber optics provide a number of advantages such as a high signal bandwidth and a low level of radiated noise.
However, there are still a number of limitations associated with the use of light transmissive materials in conjunction with electronic circuitry. In particular, the presently available technology for interconnecting circuit elements is not entirely satisfactory in a number of applications.
One type of connector that is presently utilized to connect optoelectronic devices to fiber optic media is called the "pigtail" connector. In one embodiment of the pigtail connector, an integrated circuit configured in a dual in-line package is contained within a housing, and mounted to a printed circuit board. Below the integrated circuit, and within the housing are contained light-producing or light-detecting elements coupled to leads of the integrated circuit. The light-producing element, for example, can be a semiconductor laser. These elements are typically attached to a collimator such as a collimating fiber, which resides within the housing. The collimating fiber extends to the edge of the housing, where the collimator is adjoined to an external fiber optic line that carries signals to and/or from the integrated circuit.
Although useful in many applications, pigtail connectors have a number of disadvantages when used in other applications. For example, pigtail connectors are sometimes difficult to repair since they are intended to permanently connect an optoelectronic device to a particular fiber. In addition, the fiber can not be readily disconnected from the housing. Thus, the integrated circuit can not be easily connected to a different fiber. Likewise, the fiber cannot be replaced without difficulty.
Furthermore the capability of pigtail connectors is limited to interfacing an integrated circuit with a small number of fiber optic lines. In fact, a single pigtail connector is only capable of connecting to a single fiber optic line.
Another approach that has been used to connect optoelectronic devices to fiber optic media has been the integration of light transmissive material into various layers of printed circuit boards. An example of this technology is found in U.S. Pat. No. 4,732,446 to Gipson et al. entitled "Electrical Circuit and Optical Data Buss."
Gipson et al. use a printed circuit board having embedded therein an optical data buss comprised of optical fibers. Holes are provided in the printed circuit board at predetermined locations to receive chip carriers. The chip carriers have transparent regions for interfacing with the optical fibers. The chip carriers also have means for receiving electrical signals from electrically conductive tracks embedded in the printed circuit board. Gipson et al. utilize an external connector to supply electrical power, electrical data input, and optical data to the printed circuit board.
Although the integration of fiber optic busses into printed circuit boards has been satisfactory for some purposes, there are a number of disadvantages to this approach in some applications. For example, if the fibers are somehow damaged, they are not easily repaired since they are permanently integrated into the printed circuit board. Furthermore, due to the permanent positioning of the data busses, the Gipson et al. system may not be as versatile as desired since the data busses cannot be easily re-routed to achieve diverse interconnections. In addition, two circuits within the same printed circuit board are not easily interconnected with the Gipson et al. approach.
Another approach used to connect optoelectronic devices to fiber optic media is called "lateral coupling". One form of lateral coupling is described in "An Effective Lateral Fiber-Optic Electronic Coupling and Packaging Technique Suitable for VHSIC Applications", by Hartman et al., in the Journal of Lightwave Technology, Vol. LT-4 No. 1, Jan. 1986.
Hartman et al. describe a packaged fiber pigtail. In the Hartman et al. package, a coupler fiber with a 45.degree. polished and mirrored Aluminum surface is installed within an optical receiver integrated circuit package assembly. The integrated circuit is mounted on a pedestal spacer within a rectangular metal flatpack, and the coupler fiber is attached to the integrated circuit with an ultraviolet curing epoxy.
A similar approach is discussed in the IBM Corp. publication "Optical Fiber Coupling Approaches for Multi-Channel Laser and Detector Arrays" by Jackson et al. The Jackson et al. article involves a fiber optic coupling approach for aligning four multi-mode fibers with a four-channel GaAs laser and detector array. The fibers are placed horizontally to the GaAs chip and the ends of the fibers are beveled at an angle of 35.degree. to reflect light downward onto the photodetectors. Alternatively, the beveled surface is metalized and the bevel angle is 45.degree.. Solder or epoxy is used to bond the fibers into grooves formed in a silicon substrate.
An approach like that of Hartman et al. and Jackson et al. is shown in Bellcore publication "Board Level High Speed Photonic Interconnections: Recent System Developments", by Lalk et al. Lalk et al. utilize an optical link to couple from a small area photodetector to a large rectangular waveguide made from ultraviolet-curable adhesives. The waveguide functions analogously to an optical fiber and the coupling is achieved through the use of a tapered fiber beveled at 35.degree..
In addition, Lalk et al. describe optical coupling between a laser diode and a waveguide utilizing a lensed fiber that is mounted in a hole drilled in the package wall. The fiber lens is utilized to collimate the light when it exits the package.
Although the connections disclosed by Hartman et al., Lalk et al., and Jackson et al. are useful for some purposes, these arrangements have several limitations. Specifically, these couplers involve permanent connections between a particular circuit and one or more fibers. Thus, the interconnections between circuit components cannot be varied to achieve diverse results.
Another limitation of these systems is that a substantial amount of time is required to achieve the desired connection. As discussed hereinabove, these couplers involve the use of epoxy or solder to attach the fiber to a substrate. Furthermore, the beveled surface of the fiber must be accurately aligned with the emitter or detector in order to achieve a connection with minimal losses.
Thus, the approaches of Hartman et al., Lalk et al., and Jackson et al. have certain disadvantages. In contrast to these approaches, various modular connectors have been used. A typical modular connector includes a light-producing or light-emitting device having two attachments. The first attachment can be connected to a circuit element such as an integrated circuit. The second attachment can be mated with a fiber optic line having a fixture that corresponds to the second attachment. Thus, the modular connector can be utilized as an interface between a circuit such as an integrated circuit and a fiber optic line.
As an example, Honeywell Corp. produces a number of modular connectors. Fixtures such as those described hereinabove are manufactured by corporations such as AT&T and GTE, and include numerous arrangements such as "FC", "BICONIC", "SMA", "ST", "D4", and "FC-PC", wherein ST is a registered trademark of AT&T Technologies.
Although useful for some purposes, these connector packages are not entirely satisfactory. In particular, the modular connectors are typically large, although compactness is often a primary consideration in the design of a printed circuit board.
Another limitation of the modular connectors is that each connector is only capable of attaching to a single fiber. Thus, one modular connector is required for each integrated circuit lead to be interfaced with a fiber optic line. As a result, modular connectors cannot conveniently be utilized in conjunction with fiber optic transmission lines having a plurality of fibers, such as fiber "ribbons".
Modular connectors are also limited in that they may not be easily connected to integrated circuits having a large number of leads such as pin grid arrays. Due to the size of the modular connectors, and the requirement for a separate modular connector for each lead to be accessed, circuits such as pin grid arrays cannot be feasibly interfaced with a plurality of fiber optic lines using modular connections.